Controlled catalytic decomposition of monopropellants is a preferred method for rocket propulsion and gas generation for rotary power in auxiliary power units. A monopropellant with a long standing history of usage is hydrogen peroxide H.sub.2 O.sub.2. However, hydrogen peroxide suffers from lack of storage stability and is detonable in high concentrations.
In addition to propellant stability and shock sensitivity issues, there are also volatility and toxicity concerns with respect to many conventional propellants. For example, hydrazine-based monopropellants, which currently serve a wide range of rocket propulsion and auxiliary/emergency power unit gas generator applications in satellites, space shuttles and high performance fighter aircraft, can pose a health risk to service personnel accidentally exposed to hydrazine vapors. Accordingly, there is a need by the propellant community for relatively insensitive propellants characterized by relatively low toxicity and volatility, and new processes for their deployment as monopropellants.
Hydroxylammonium nitrate (so-called "HAN"), which is the nitrate salt of hydroxylamine, is known to be an excellent insensitive oxidizer for liquid gun propellants, as disclosed, for example, in U.S. Pat. No. 4,968,394 which is more specifically directed at a method for reducing excess nitric acid in aqueous hydroxylammonium nitrate solutions.
Of course, as with any propellant, there are risks associated with HAN and related salts. For example, Ullman's Encyclopedia of Industrial Chemistry (5th Rev. ed. Vol. A13, 528, 1989) reports that hydroxylammonium salts decompose on heating, and runaway exothermic decomposition can take place as a result of local heating. Such uncontrolled decomposition can be also promoted by even trace amounts of unwanted heavy metal impurities, particularly, copper, copper-containing alloys and copper salts. These contaminants act as catalysts, typically homogeneous catalysts. This type of catalysis is very undesirable and not a subject of the current patent. In contradistinction, controlled and desirable on-command on-off catalytic decomposition of HAN-based propellants is possible through use of heterogeneous catalysts. Thermal decomposition products of hydroxylammonium sulfate are reported to be sulfur dioxide, dinitrogen monoxide (nitrous oxide), water and ammonium sulfate; the chloride similarly decomposes to hydrogen chloride, nitrogen, water and ammonium chloride. Hydroxylammonium salts react with alkali to give the easily decomposed hydroxylamine free base. With nitrites, decomposition to dinitrogen monoxide occurs. Heating of aqueous solutions of hydroxylammonium salts in the absence of catalysts for long periods at 80.degree. C. leads to slow decomposition. Oxidation of hydroxylammonium salts leads, depending on the oxidant and reaction conditions, to nitrogen compounds in various oxidation states. All these reactions are greatly accelerated in the presence of a catalyst.
Various processes for destroying waste HAN and related salts are known in the art. For example, U.S. Pat. No. 4,927,542 teaches a process for eliminating hydroxylammonium salts from acidic solutions by the use of manganese (IV) oxides. This process, however, introduces undesirable metal ions into the solutions and requires several treatment steps to achieve decomposition to where the effluent can be discharged or disposed of safely, for example, in a navigable waterway.
As another illustration of a process for the disposal of hydroxylammonium salts, U.S. Pat. No. 5,062,966 discloses a process for reacting an aqueous solution of a hydroxylammonium compound which comprises raising the pH of the solution to about 8 or higher and reacting the alkaline solution with a source of hypohalite ions. Unfortunately, the process of the U.S. Pat. No. 5,062,966 requires the use of consumable chlorine or other halogen and tends to produce undesirable halogen by-products such as chloroform, as well as undesirable amine by-products such as nitroamines if it is applied to HAN-based propellants containing aliphatic amine nitrates.
The current invention describes a new process for decomposing HAN in a controlled manner for purposes of disposal of the HAN or to provide controlled decomposition of the HAN in its use as a monopropellant, without requiring a consumable reactant and without undesirable by-product production, which would be highly desired by the military and commercial propellants communities, as well as the waste disposal industry.
The present invention is distinctly different from the method of hybrid rocket propulsion using a highly soluble, non-hazardous hydoxylammonium salt, as described in U.S. Pat. No. 4,527,389 in that it allows the monopropellant reactor rocket engine to operate indefinitely long and continuously or repeatedly as long as a HAN/fuel solution is supplied to the reactor. In contrast, the hybrid rocket can be operated only once and its operating time is limited by the amount of solid propellant in the motor. This difference makes monopropellants useful for attitude control of satellites, divert propulsion of kinetic kill vehicles and auxiliary power supplies which cannot be powered by a hybrid rocket. At the start of a hybrid rocket, liquid oxidizer and solid propellant are in separate containers, whereas in a monopropellant the HAN oxidizer and fuel are dissolved intimately in a solvent and contained in a common tank.
The key to successful operation of a monopropellant-powered rocket is the catalyst, whereas the hybrid rocket motor in U.S. Pat. No. 4,527,389 operates without a catalyst.